From the introduction of Christianity into Ireland in the fifth century to the arrival of the Vikings in the ninth, the attitudes of the Irish Church towards bloodshed and violence changed considerably. The moral code of pacifism and non-violence, especially towards other Christians, advocated by the Christian Churches came into direct confrontation with the violent necessities of secular life when the Roman Empire adopted the new faith as its state religion. Further difficulties arose when the Roman Empire gave way to Germanic kingdoms, and when the Christian faith began to make its way out to lands unconquered by Rome, such as Ireland; challenged by cultures in which honour and violence were part of the social fabric, and by the idea that victory in battle demonstrated divine favour, the Church had to both integrate itself into these new lands and try to draw them closer to the Christian ideal. Penance for the sins committed in life could be undertaken, but it was an arduous and humiliating process, such that many did not seek redemption until near death, an attitude which did not rest easy with the Church. The monastic system of penance, fixed in term and confessed in private, became available to the laity in the British Isles, a seismic shift which would allow a layman a new avenue to atone for sins of bloodshed, from murder to killing in war. It has, however, been questioned as to whether such penance was widely available to the laity as a whole or only to a specific group from among them. This thesis will explore how this changing attitude towards violence within the Irish Church demonstrates that this new form of penitential practice was indeed available to the whole laity through examining the development in nuance concerning the various sins of bloodshed across not only the Irish Penitentials, but hagiography, canon law, secular law, narratives, and other texts.